Builtin types

Generated Mon 05 Jan 2026 06:11:07 UTC

Exception

All exceptions have readable value and errno attributes, not just StopIteration and OSError.

Cause: MicroPython is optimised to reduce code size.

Workaround: Only use value on StopIteration exceptions, and errno on OSError exceptions. Do not use or rely on these attributes on other exceptions.

Sample code:

e = Exception(1)
print(e.value)
print(e.errno)

Exception chaining not implemented

Sample code:

try:
    raise TypeError
except TypeError:
    raise ValueError

User-defined attributes for builtin exceptions are not supported

Cause: MicroPython is highly optimized for memory usage.

Workaround: Use user-defined exception subclasses.

Sample code:

e = Exception()
e.x = 0
print(e.x)

Exception in while loop condition may have unexpected line number

Cause: Condition checks are optimized to happen at the end of loop body, and that line number is reported.

Sample code:

l = ["-foo", "-bar"]

i = 0
while l[i][0] == "-":
    print("iter")
    i += 1

Exception.init method does not exist.

Cause: Subclassing native classes is not fully supported in MicroPython.

Workaround: Call using super() instead:

class A(Exception):
    def __init__(self):
        super().__init__()

Sample code:

class A(Exception):
    def __init__(self):
        Exception.__init__(self)


a = A()

bytearray

Array slice assignment with unsupported RHS

Sample code:

b = bytearray(4)
b[0:1] = [1, 2]
print(b)

bytes

bytes objects support .format() method

Cause: MicroPython strives to be a more regular implementation, so if both str and bytes support __mod__() (the % operator), it makes sense to support format() for both too. Support for __mod__ can also be compiled out, which leaves only format() for bytes formatting.

Workaround: If you are interested in CPython compatibility, don’t use .format() on bytes objects.

Sample code:

print(b"{}".format(1))

bytes() with keywords not implemented

Workaround: Pass the encoding as a positional parameter, e.g. print(bytes('abc', 'utf-8'))

Sample code:

print(bytes("abc", encoding="utf8"))

Bytes subscription with step != 1 not implemented

Cause: MicroPython is highly optimized for memory usage.

Workaround: Use explicit loop for this very rare operation.

Sample code:

print(b"123"[0:3:2])

dict

Dictionary keys view does not behave as a set.

Cause: Not implemented.

Workaround: Explicitly convert keys to a set before using set operations.

Sample code:

print({1: 2, 3: 4}.keys() & {1})

float

uPy allows implicit conversion of objects in maths operations while CPython does not.

Workaround: Objects should be wrapped in float(obj) for compatibility with CPython.

Sample code:

class Test:
    def __float__(self):
        return 0.5


print(2.0 * Test())

uPy and CPython outputs formats may differ

Sample code:

print("%.1g" % -9.9)

int

bit_length method doesn’t exist.

Cause: bit_length method is not implemented.

Workaround: Avoid using this method on MicroPython.

Sample code:

x = 255
print("{} is {} bits long.".format(x, x.bit_length()))

No int conversion for int-derived types available

Workaround: Avoid subclassing builtin types unless really needed. Prefer https://en.wikipedia.org/wiki/Composition_over_inheritance .

Sample code:

class A(int):
    __add__ = lambda self, other: A(int(self) + other)


a = A(42)
print(a + a)

list

List delete with step != 1 not implemented

Workaround: Use explicit loop for this rare operation.

Sample code:

l = [1, 2, 3, 4]
del l[0:4:2]
print(l)

List slice-store with non-iterable on RHS is not implemented

Cause: RHS is restricted to be a tuple or list

Workaround: Use list(<iter>) on RHS to convert the iterable to a list

Sample code:

l = [10, 20]
l[0:1] = range(4)
print(l)

List store with step != 1 not implemented

Workaround: Use explicit loop for this rare operation.

Sample code:

l = [1, 2, 3, 4]
l[0:4:2] = [5, 6]
print(l)

memoryview

memoryview can become invalid if its target is resized

Cause: CPython prevents a bytearray or io.bytesIO object from changing size while there is a memoryview object that references it. MicroPython requires the programmer to manually ensure that an object is not resized while any memoryview references it.

In the worst case scenario, resizing an object which is the target of a memoryview can cause the memoryview(s) to reference invalid freed memory (a use-after-free bug) and corrupt the MicroPython runtime.

Workaround: Do not change the size of any bytearray or io.bytesIO object that has a memoryview assigned to it.

Sample code:

b = bytearray(b"abcdefg")
m = memoryview(b)
b.extend(b"hijklmnop")
print(b, bytes(m))

str

Start/end indices such as str.endswith(s, start) not implemented

Sample code:

print("abc".endswith("c", 1))

Attributes/subscr not implemented

Sample code:

print("{a[0]}".format(a=[1, 2]))

str(…) with keywords not implemented

Workaround: Input the encoding format directly. eg print(bytes('abc', 'utf-8'))

Sample code:

print(str(b"abc", encoding="utf8"))

str.ljust() and str.rjust() not implemented

Cause: MicroPython is highly optimized for memory usage. Easy workarounds available.

Workaround: Instead of s.ljust(10) use "%-10s" % s, instead of s.rjust(10) use "% 10s" % s. Alternatively, "{:<10}".format(s) or "{:>10}".format(s).

Sample code:

print("abc".ljust(10))

None as first argument for rsplit such as str.rsplit(None, n) not implemented

Sample code:

print("a a a".rsplit(None, 1))

Subscript with step != 1 is not yet implemented

Sample code:

print("abcdefghi"[0:9:2])

tuple

Tuple load with step != 1 not implemented

Sample code:

print((1, 2, 3, 4)[0:4:2])